1. Field of the Invention
The present invention relates generally to catheters and more specifically, to an improved catheter which incorporates shape memory alloy structures comprising an expandable catheter housing wall and a shutter mechanism for enhancing the operation of the catheter.
2. PRIOR ART
Angioplasty, a cardiological therapeutic procedure for removing plaque deposits in blocked arteries has become increasingly popular among cardiovascular surgeons as a substitute for bypass or vascular graft surgery among some patients. Currently, balloon and laser angioplasty are the most frequently used clinical procedures. The balloon simply pushes the clotted vessel locally to enlarge the narrowed artery. The laser method literally evaporates the deposited plaque. However, neither of these procedures is effective in cases in which the plaque has been calcified. Recently a new type of catheter called the "Simpson Atherocath" has been clinically tested and has shown promising results. This new catheter utilizes a rotational cutter which removes atheroma through a longitudinal cut-out on a cylindrical housing located at the distal end of a catheter. One major disadvantage of the Simpson Atherocath is that the catheter has to be removed from the patient's blood vessel several times during the catheterization process to empty the plaque collection chamber when it is full. Such repeated removal and reinsertion of the catheter into the patient's blood vessel, causes a high level of discomfort to the patient who has to lie motionless for several hours on an operating table. In addition, it also increases the substantial risk of developing other complications during the angioplasty procedure. A detailed description of the atherocatheter and its components is provided in U.S. Pat. Nos. 4,669,469; 4,461,305; 4,616,648; and 4,616,652.
Shape memory alloy material has been known for some time. By way of example, in an article entitled "Shape Memory Alloys" written by L. McDonald Schetky and appearing in Scientific American, November 1979 beginning at page 74, the author thoroughly explains the mechanical properties of these alloys and indicates that they first came to worldwide attention in 1962. As further explained in that article the phenomenon of shaped memory alloys is dependent upon the characteristic of certain materials to possess a martensite crystal structure derived from a parent crystal phase when treated by a certain combination of stress and temperature processing. This martensitic crystal phase can then be transformed back into its parent crystal phase thermoelastically by elevating the temperature of the material. In addition, for certain "two-way" shape memory alloys, this phase transformation can be reversed by lowering the temperature of the material. As a result of these phase transformations, the material can be induced to change its shape in a carefully controlled and reversible manner by simply controlling the temperature of the material to induce the aforementioned phase transformations. The article by Schetky illustrates a particular exploitation of this phenomenon for automatically deploying an antenna for a spacecraft. An updated description of the "Shape Memory Effect Alloys" can also be found in the Encyclopedia of Materials Science and Engineering, Volume 6, pages 4365-4374 MIT Press, 1986.
Numerous patents have been issued with regard to the use of shape memory alloy materials in the medical device industry. by way of example, U.S. Pat. Nos. 4,411,655 and 4,665,906; Japanese Pat. Nos. 59/067968; 61/193670 and 87/020827; and European Pat. Nos. 145,166 and 244,818 disclose the use of shape memory alloy materials in catheter applications. However, the use of the multi-purpose structure described herein for catheter applications has not been disclosed previously in any prior art known to the applicant.